1. Field of the Invention
The present invention relates generally to a high-frequency induction heating system which is applicable to electromagnetic cooking heaters, for example. More specifically, the invention relates to circuit protection for a high-frequency induction heating system which protects the circuitry of the induction heating system from high-voltage noise which may be superimposed on the power supply voltage.
2. Description of the Prior Art
As is well known, a high-frequency induction heating system, such as that utilized in electromagnetic cooking heaters, comprises an induction element. The induction element typically comprises an induction coil supplied with a relatively high-frequency AC signal in order to generate an alternating magnetic flux. A conducting material, such as an iron pan or the like, is placed within the flux, whereby eddy currents are induced in the material, and the induced eddy currents produce heat. The amount of heat induced is a function of the frequency of the flux as well as its intensity. The intensity of the flux is a function of the power supplied to the induction element, while the frequency at which the flux changes is determined by opening and closing a switching device which is connected in series with the element. Thus, the amount of heating which is produced by the induction heating system is controlled by controlling the frequency at which the switching device operates and by controlling the amount of power which is supplied to the heating device.
Such high-frequency induction systems have been disclosed in the U.S. Pat. No. 4,161,022, issued to Kanazawa et al, on July 10, 1979, and in the Japanese Patent First Publication (Tokkai) Showa No. 54-31646, published on Mar. 8, 1979, both of which have been assigned to the owner of the present invention.
In such high-frequency induction heating systems, power is supplied by a commercial AC power source. Noise is commonly superimposed on the power supply from the power source. Noise spikes, however, apply significant loads on the switching devices used in the induction heating system. In particular, in areas where the commercial AC power is at a relatively high voltage, e.g. 220 V, the load exerted on the switching device by noise may be heavy enough to damage it.
In practice, AC voltage from a commercial AC power source is supplied to a rectifying circuit via a power switch for rectification. The rectified output of the rectifying circuit is supplied to a smoothing circuit for conversion to DC. An induction coil and the switching device which may comprise a switching transistor are connected in series across the output terminals of the smoothing circuit. A capacitor for resonance is connected to one output terminal of the smoothing circuit in parallel with the induction coil. A damper diode is connected in parallel across the collector and emitter of the switching transistor.
When a switching pulse is applied to the switching transistor, a saw-tooth waveform current passes through the collector electrode of the switching transistor, and also a half-wave rectified voltage appears at the the collector electrode of the switching transistor due to the resonance of the induction coil and the capacitor. Therefore, if a cooking pan were placed near the induction coil at this time, the pan would be heated by eddy-current losses in the magnetic flux generated by the induction coil, so that cooking could be carried out.
When a noise spike is superimposed on the AC power supply, such as is commonly generated when another relatively heavy load, such as a refrigerator, is switched on, the collector voltage can exceed the collector break-down voltage (generally about 1000 V) of the switching transistor. In such a case, the switching transistor can easily be destroyed.
Break-down of the switching transistor can be prevented by using a transistor with a sufficiently large break-down voltage. However, the switching properties of a switching transistor are generally inversely related to the break-down voltage of its collector, so that a switching transistor with a collector having a high break-down voltage will have disadvantageous switching properties and will not be suitable for high-frequency induction heating.
Furthermore, although switching transistors with both superior switching characteristics and a high collector break-down voltage are available on the market, such transistors are bulky and expensive. Therefore, from the point of view space and cost, transistors with superior switching characteristics and collector break-down voltage cannot be used.